Cassini plunge: Can you fly through a gas giant?

(Artist's impression of Cassini near Saturn. Credit: Nasa)

On Wednesday, the Cassini space probe began its planned but fateful descent towards Saturn, a plunge at a speed of more than 110,000km/h (70,000mph) from which it will never return. But given that planet Saturn is just a giant ball of gas, what will happen to the truck-sized craft?

The intrepid craft – a joint project of Nasa, ESA (the European Space Agency) and the Italian Space Agency - will make a total of 22 dives through the 2,000 km gap between the gas giant and its ring system, before descending into its atmosphere in mid-September. Each of the manoeuvres will take the probe high above and far below Saturn. The first pass through the system was successful, and the hope is that Cassini will survive all other fly-bys near the rock and ice particles making up the rings, and beam back to Earth images of Saturn that are really up close and personal, and provide valuable science data.

“There is a very good probability that Cassini will survive its passage between Saturn and its rings,” says Scott Edgington, Cassini deputy project scientist at Nasa’s Jet Propulsion Laboratory in Pasadena, CA. “Imaging data collected by Cassini and our best models, verified with Cassini's plasma instrument data, indicate that this region is very clear of icy particles.”

The major risk of getting hit by dust and particles is when the spacecraft flies through the ring plane – the equatorial plane of Saturn where bigger particles could be,” says Sean Hsu, planetary scientist at the University of Colorado, Boulder.

On its way down, the craft will get to ‘taste’ some of the planet’s atmosphere during the last few orbits before it goes too deep. The crafts’ operators have made sure that Cassini will not go too deep until the end of the mission, and not too far away to touch Saturn's innermost D-ring, to prevent or minimise impacts – because even a single particle the size of a sand corn could fatally damage it.

(Credit: Nasa)To shield the craft, its controllers at Nasa opened Cassini's dish-shaped high-gain antenna, which measures 4m across in the direction of travel and used it as “the best – and probably the only – shield Cassini has,” says Hsu. It’s impossible to know whether the probe survives. “But this is the beauty of the planning – Cassini is running out of fuel, so it is time to do some rather risky moves – to explore the region that hasn’t be visited before,” he adds.

One of the aims during these final five months of the probe’s life is to determine the mass of the rings, made out of nearly pure water-ice, from which researchers can determine their age. To do so, scientists will be studying how Cassini’s speed changes as it flies through Saturn’s gravity field.

But what actually happens to a spacecraft when it approaches a gas planet? Won’t it just fall straight through?

Saturn is called a ‘gas giant’ because it is made mostly of hydrogen and helium, says Elizabeth Bailey, a planetary scientist at the California Institute of Technology in Pasadena. “But the pressure and temperature deep in gas giant planets are immensely greater than what we encounter in everyday life on Earth.”

On Saturn, just like on Earth, the atmosphere gets denser the lower the altitude. But given the size of the planet, it means that deep inside Saturn, hydrogen and helium are nothing like the light, airy gases we are familiar with. Instead, they take the form of “a dense fluid, and it's very hot,” says Bailey.

Because of the huge speed difference between the spacecraft and Saturn – probably about 30 km/s - Cassini will heat up and eventually get ablated, “because of the friction between the spacecraft and the gas molecules in the atmosphere of Saturn, given that you go deep enough,” says Hsu. Simply put, he adds, Cassini will end up “as a splendid meteor show and essentially become part of Saturn’s atmosphere”.

Even if Cassini doesn’t break up into pieces and melt along the way due to friction and heating by the atmosphere (similar to a meteorite here on Earth), “it would soon encounter atmosphere thicker than mushy peas and the increasing pressure would eventually crush it,” explains Edgington.

It feels like a sad end, says Bailey, but “it would be bad practice to let it age until it loses all function. For example, it would be unfortunate if it crashed into one of Saturn's pristine moons.”

So what has the little spacecraft done for humanity? “The Cassini discoveries have significantly altered our view on the solar system bodies, and changed the direction of planetary science,” says Hsu. Let’s have a look.

1. TITAN

(Credit: Nasa)When Cassini first arrived at the outskirts of Saturn on 25 December 2004, it had a passenger: Huygens lander. The two parted ways in January 2005, and Huygens landed on Titan, the largest of Saturn’s many moons. Huygens’ descent took two hours and 27 minutes, and the images it sent back revealed the moon to be very similar to Earth before life evolved. There is liquid on Titan, with lakes and clouds, but it’s not water – the gas giant’s companion is covered in hydrocarbons, like methane and ethane.

“Before this mission, nobody knew what would be found beneath the hazy atmosphere of Titan,” says Bailey. “People were making bets about what the Huygens lander would find. And the result was as fascinating as could be.”

Over the years, Cassini visited Titan a few more times. Thanks to its fly-bys, researchers could determine the wild mix of molecules in the moon’s complex atmosphere. Close to the surface, methane, ethane, and other organics condense and rain down – and, say researchers, it’s possible that other prebiotic chemistry can take place at the surface as well. Cassini also spotted the presence of a subsurface ocean on the moon, probably as salty as our Dead Sea.

2. UNPRECEDENTED CLOSE-UPS OF JUPITER

During its multiple fly-bys of Jupiter (before travelling on to Saturn), Cassini took spectacular pictures of the gas giant – the most detailed colour photos of it ever recorded.

"Everything visible on the planet is a cloud," Nasa writes in a blog post. "The parallel reddish-brown and white bands, the white ovals, and the large Great Red Spot persist over many years despite the intense turbulence visible in the atmosphere. These clouds grow and disappear over a few days and generate lightning. Streaks form as clouds are sheared apart by Jupiter's intense jet streams that run parallel to the colored bands. The prominent dark band in the northern half of the planet is the location of Jupiter's fastest jet stream, with eastward winds of 480 kilometers (300 miles) per hour. Jupiter's diameter is eleven times that of Earth, so the smallest storms on this mosaic are comparable in size to the largest hurricanes on Earth.

Unlike Earth, where only water condenses to form clouds, Jupiter's clouds are made of ammonia, hydrogen sulfide, and water. The updrafts and downdrafts bring different mixtures of these substances up from below, leading to clouds at different heights. The brown and orange colors may be due to trace chemicals dredged up from deeper levels of the atmosphere, or they may be byproducts of chemical reactions driven by ultraviolet light from the Sun. Bluish areas, such as the small features just north and south of the equator, are areas of reduced cloud cover, where one can see deeper.”

3. SATURN'S HURRICANE

In 2010, Cassini witnessed a violent storm on Saturn. It usually occurs every 30 years, but this time it started a decade early, which gave Cassini unexpectedly a premier seat. The storm lasted several months and engulfed the whole planet, before finally subsiding when its head collided with its tail. During the storm, Cassini recorded Saturn’s temperatures soaring to values never before measured on any planet, and detected totally new molecules in the gas giant’s upper atmosphere.

4. ENCELADUS AND LIFE

(Credit: Nasa)When flying by Saturn’s moon Enceladus, Cassini discovered tidally driven plumes of water erupting through the icy surface of its oceans. In other words, this moon could potentially harbour life. “We still don’t understand how a tiny object like Enceladus can still be active,” says Hsu. “Small bodies should lose their initial heat from the accretion phase and the radiogenic output is way lower than the energy required for the observed activities.”

More recently, Cassini also found that Enceladus might have a water ocean beneath its icy outer shell and conditions favourable for microbes, making this little moon an ever more exciting place to visit in the future. “I consider Cassini’s discovery of Enceladus' global liquid water ocean with both hydrothermal activity and basic life providing molecules as one of Cassini's most important discoveries,” says Edgington. “The discovery of Titan’s global ocean and atmosphere teeming with prebiotic molecules is a very close second.”

5. YING-YANG OF IAPETUS

Yet another one of Saturn’s many companions, Iapetus, is the planet’s third-largest moon. It’s different from others though: it two-faced surface puzzled researchers for over 300 years. Cassini found that the effect is due to the reddish dust in the moon’s orbital path that falls on Iapetus’ face. The dust makes some areas darker – and they then absorb energy and become warmer, while those without the dust stay cooler.

6. SATURN’S RINGS

Cassini took images of Saturn’s rings – A LOT of images. Over the years, the pictures have helped scientists monitor changes in the gas planet’s dynamic ring system. The probe spotted propeller-like formations and saw what could have been the possible birth of a new moon.

Once every 15 or so years, the edge of Saturn’s ring plane gets illuminated by sunlight, while northern and southern sides of the rings get very little of it. When it happened, the probe measured the long shadows that were produced, and scientists were able to calculate the heights of structures within the rings.

7. SATURN’S UNKNOWN MOONS

(Credit: Nasa)We knew Saturn had many moons – but we didn’t know that there were THAT many of them. Cassini found seven previously unknown satellites, including Methone, Pallene, Polydeuces, Daphnis, Anthe and Aegaeon. The seventh moon, discovered in 2009, is currently named S/2009 S 1 and is only 984 feet in diameter. Around 150 moons and moonlets have been detected in total. Only 62 have confirmed orbits though, and only 53 have been given official names.

There’s much more, of course. As JPL puts it on their website, “Cassini is, in a sense, a time machine. It has given us a portal to see the physical processes that likely shaped the development of our solar system, as well as planetary systems around other stars”.